Abstract: We consider the most general asymptotically flat boundary conditions in
three-dimensional Einstein gravity in the sense that we allow for the maximal
number of independent free functions in the metric, leading to six towers of
boundary charges and six associated chemical potentials. We find as associated
asymptotic symmetry algebra an isl(2)_k current algebra. Restricting the
charges and chemical potentials in various ways recovers previous cases, such
as BMS_3, Heisenberg or Detournay-Riegler, all of which can be obtained as
contractions of corresponding AdS_3 constructions. Finally, we show that a flat
space contraction can induce an additional Carrollian contraction. As examples
we provide two novel sets of boundary conditions for Carroll gravity.